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John Adolphus Flemer.

An elementary treatise on phototopographic methods and instruments, including a concise review of executed phototopographic surveys and of publicatins on this subject online

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Online LibraryJohn Adolphus FlemerAn elementary treatise on phototopographic methods and instruments, including a concise review of executed phototopographic surveys and of publicatins on this subject → online text (page 27 of 33)
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appearing as light, others in half-light, still others in middle
tint, half dark, and dark.

Practical experience, on the other hand, teaches that the
Tarious blues and dark greens of the chromatic scale appear darker
to the eye than the yellows, the reds, and the lighter shades of
green, yet, when a photographic plate is exposed in the camera
to both, the combined action of the luminous and actinic light-
Tays of an 'illuminated landscape, the actinic action of the blue
rays will be more intense on the sensitized film than that of the
light-green, the yellow, orange, and red rays. In order to obtain,
therefore, a clear and well-defined picture of the violet and blue-
colored parts, the exposure will have to be stopped long before
the parts having shades of light green, yellow, orange, and red
have been reproduced on the negative. The resulting mono-
chrome picture will have a scale of but three-tone gradations
light, half-light, and dark instead of the scale of five gradations
of tone mentioned above.

Thus it may happen, when an ordinary dry-plate is exposed



ORTHOCHROMATIC DRY-PLATES AND RAY-FILTERS. 339

in the camera, the sky and blue- tinted parts in general will be
overexposed if the exposure had been timed correctly for the
green, yellow, orange, and red-tinted parts. For the use of the
phototopographer who desires negatives showing also the dis-
tant details of the landscape clearly and well denned the ordinary
dry-plate is inadequate.

II. Orthochromatic Dry-plates and Ray-filters.

In the preparation of the sensitive film of the " isochromatic *
(rendering all color values evenly well), or " Orthochromatic "
plates (rendering the color values correctly), it has been the
aim to make them equally sensitive to the actinic action of all
color- rays, so that during a properly timed exposure all colors
of the subject may be represented upon the finished negative
equally correct regarding their respective tints and light values.
As yet, attempts in this direction have been only partially suc-
cessful, however. Orthochromatic plates are indeed . made
more sensitive to the 'actinic effects of red, yellow, and light-green
rays, but the blue rays remain, even with these plates, consider-
ably more active than the reds and yellows, and to retard their
chemical action still more, a so-called ''.color-screen," or " ray-
filter," is interposed between the plate and the subject. A suit-
able combination of Orthochromatic plate and color-screen makes
it possible to reproduce landscapes (and colored objects) in
better harmony regarding chromatic values, reducing the actinic
power of the rays of long wave-lengths and increasing it for the
rays of the less refractive end of the solar spectrum.

At present the Orthochromatic plates are prepared by impart-
ing color sensitiveness to the gelatino-bromide-silver emulsion
of the ordinary dry-plates by the addition of certain color ingre-
dients or " optical sensitizers," like erythrosine, cyanine, rhoda-
mine, eosine, etc. The nearer these optical sensitizers approach
a blue shade of color, the more sensitive the plate will become
for light-rays of the less refractive end of the solar spectrum.



340 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

The addition of erythrosine is said to increase the sensitizing
action of the emulsion for light reds, while rhodamine increases
the same for light greens, extending well toward the yellow and
light orange. Tetrachlor-tetraethyl-rhodamine-chlorhydrate im-
parts a more powerful sensitizing action for the orange yellow,
and green tints. Cyanine has a greater orange sensitiveness than
either of the ingredients named, excepting, perhaps, the last-
mentioned dye, but the others have the advantage of not materi-
ally reducing the general speed or sensitiveness of the plate.
Valuable experiments in this direction establishing the fore-
going facts have been made by Dr. Eder, Valenta, Mallmann,
Scolik, Schumann, Obernetter, and others, who have published
formulas for the various optical sensitizers that they recommend
individually.

A. Color-screens, or Ray- filters.

The general introduction of color-sensitive plates has been
somewhat retarded on account of the necessity of a materially
increased length of exposure when using a color-screen, pre-
cluding the use of this combination for all instantaneous work.

For phototopographic purposes ray-filters are used from a
bright-yellow to a deep-orange tint, varying with the character
of the plates and lenses used. In the Canadian surveys, for
instance, an orange- colored filter was used with the Zeiss Anas-
tigmat Lens No. 3, Series V, together with Edwards' Iso-
chromatic Medium Plate, while a light-yellow screen (Car-
butt's) gave good results in connection with Dallmeyer's W. A.
Lens and Carbutt's Orthochromatic Plates (sensitometer No. 23),
for the topographic reconnaissance, made by the U. S. Coast
and Geodetic Survey, in S.E. Alaska.

Carbutt's (pale-yellow) screens are composed of two thin
piano-parallel crystal-glass plates cemented together with bal-
sam and having the color matter between the plates. They are
2 i or 3i inches square and can be placed in grooved pieces of
wood suitably attached to the back of the lens board. The



ORTHOCHROMATIC DRY-PLATES AND RAY-FILTERS. 341

screen should always be in position when focusing; when not
in use it should be kept in a box protected against light. With
Carbutt's light color-screen (yellow) the action of the chromatic
rays begins between the Fraunhofer lines C and D and it ends
between the lines E and F.

Dallmeyer's yellow screens are fitted into metal settings
which may be attached to the lens mount, close to one side of
the diaphragm when a lens doublet is used.

The Bausch and Lomb filter is in the form of a hollow glass
cylinder that may be filled with variously colored liquids to suit
different optical demands. The piano-parallel ends of these
cylinders are made of optically worked glass and the whole is
incased in a metal ring that fits over the lens mount.

Theoretically, the color-shade of the screen should decrease
in intensity, from the center toward the edge, in the same ratio
as the intensity of the illumination of the plate in the camera
decreases from the center toward the margin and its form should
be spherical, its center of curvature being in the second nodal
point of the camera-lens. For all practical purposes, however,
distortion, due to the use of a piano-parallel screen, placed at
right angles to the optical axis, is imperceptible, particularly
when the screen is placed in the nodal plane of the lens and when
using a relatively small stop.

Terrene points in the shadows of a landscape receive but
partial illumination from the sky and atmosphere and only reflected
light from the surfaces of surrounding bodies; the rays repro-
ducing such shadows on the plate will principally belong to
the violet end of the spectrum. Hence negatives obtained
behind yellow color-screens give the shadows in exaggerated
intensity, particularly when photographing mountain views of
an Alpine character, since the rarefied air in high altitudes absorbs
less light than the air in lower altitudes. The exaggerated con-
trast between the high lights and shadows in such views makes
it desirable, to employ specially prepared plates of a thick emul-
sion coating, which have the further advantage to widen the



342 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

range of correct exposure. Thinly coated plates require accu-
rately timed exposures to avoid a characteristic flatness in their
negatives.

B. Halation.

The naturally sharp outlines defining dark sections in Alpine
views, or objects with marked contrasts, frequently appear blurred
and undefined in negatives obtained after a rather lengthy expos-
ure, such condition being caused by a reflex action of rays that
have deeply penetrated the emulsion and have been reflected
from the glass surface immediately below the plate coating.
This effect, known as " halation of the plate," may be greatly
reduced by covering the rear surface of the plate with an opaque
coating of the same refractive index as that of the glass used for
the plate. Plates provided with such protective backings are
called " non-halation " or " anti-halation " plates. The back-
ing of non-halation plates should be removed before these plates
are subjected to the developing process.

Besides the increase in range of the gradation of tints in a
monochrome reproduction of a landscape or multi-colored object,
color-screens also materially aid in the prevention of halation
when the necessity arises of having to expose a panorama plate
directly toward the sun.

To prevent possible side reflection and permit only such
rays to reach the plate which are conducive to the production
of the image, it is recommended to insert one or more diaphragms
in the camera-box and to paint these, like all other interior
surfaces of the camera-box, a dull black. Rays reflected by
the lens surfaces should likewise be excluded from the interior
of the camera. This is effected, in a measure, in the Zeiss Anastig-
mat Lens by giving the surface of the back lens a strong curva-
ture. For similar reasons certain surveying-cameras (Deville's
and that of the U. S. Coast and Geodetic Survey) are provided
with a " hood " or " lens shade."

Plates exposed in the field, when well protected against heat



COMPARATIVE LIGHT VALUES AND EXPOSURES. 343

dampness, injurious gases, and, of course, against light, both
before and after exposure, will preserve the undeveloped image
in the latent stage almost indefinitely.

HI. Comparative Light Values and Exposures.

To secure as much detail in the shadows as possible, the
plate should be given as lengthy an exposure as it will bear with-
out becoming overexposed. This length depends upon a series
of circumstances and conditions; the more important ones are:

1. The intensity of the light that reaches the plate;

2. The sensitiveness of the plate;

3. The speed of the lens;

4. The size of stop used;

5. The color and illumination of the object;

6. The character of the color-screen;

7. The distance of the object from the second nodal plane

of the lens.

Success in obtaining clear and well-defined negatives depends
largely upon properly timed exposures, demanding care, judg-
ment, and much experience, if the results are to be uniformly
successful. Various tables (and diagrams) of comparative light
values and comparative exposures have been computed from
which much information may be gained simply by inspection,
to obtain which without such aid would require much experi-
ence, time, and trouble. Such tables and diagrams, of course,
vary with the latitude of the place and its altitude above sea-
level; both, however, may be neglected for exposures made while
the sun is relatively high, say not below 45. Under this proviso
we would have to consider only the conditions of the atmosphere,
including the illumination, the hour of day, the season of the
year, the rapidity of the plate, the character of the screen, and
the lens stop, to ascertain the time for correct exposure.

Any one who has experimented with a certain brand of plate
tinder certain atmospheric conditions, in a known latitude, and



344



PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.



at a certain elevation above sea-level to ascertain the time required
for the correct exposure for a certain subject with a certain lens
and diaphragm, taking recourse to a table of comparative light
values, can readily decide what time should be given a similar
plate under the same conditions, with the same lens and dia-
phragm, at any other hour of the day and on any other day of
the year.

For a correct exposure the time given the light for action
upon the film should be inversely proportional to the intensity
of the light emanated from the subject. A subject, for instance,
requiring six seconds' exposure when the intensity of the light
be one, would, under identical conditions, require an exposure
of two seconds for a light value of .three.

The following table contains the comparative exposures for
different lens stops and for " open " and " dark " landscapes.



Number of Lens Stop ........


Comparative Exposures for Different Lens Stops.


2

F/S


4
F/8


8
F/n


16
F/i6


32

F/22


64

F/V


128

F/45


2 5 6

F/6 4




Open landscape (seconds). . . .

Average landscape with fore
and background of average
color (seconds)


1/32
1/6


1/16
i/3


1/8
3/4


i/4
li


1/2

3


i
6


2
12


4
24





The upper line of this table gives the so-called " Uniform
System " numbers of the lens stops or diaphragm apertures,
which numbers have the same ratio to each other as the areas
of their corresponding stops. The second line expresses the
ratio which the stop diameter bears to the focal length (F) of
the camera.

This table is based on a plate requiring i second's exposure
when using stop F/%2 (the diameter of this stop is ^ of the focal
length of the lens) for an open landscape. The same brand of
plate at the same date and hour and under the same conditions



COMPARATIVE LIGHT VALUES AND EXPOSURES.



345



of illumination would require ij seconds' exposure for an aver-
age landscape when using stop F/i6.

Taking the length of exposure at noon, from the middle of
April to the middle of September, as unit, the corresponding
lengths of exposures for a plate at other hours and at different
seasons of the year are given in Scott's table:



Hours


January


February


March


April


May


June


Hours


A.M.


i-iS i5-3i


1-15 15-28


1-15 15-31


1-15 15-30


i-iS 15-31


1-15 15-30


P.M.


4

















30


8


5














3 15


14 10


7


6








3


15 12


8 6


5 4


6


7





30 IS


12 7


6 4


3 2.5


2.3 2


5


8


30 15


10 6


4 3


2. 5 2


1.8 1.7


1.5 1.6


4


Q


10 6


4 4


2.1 1.8


1.7 1.6


1.5 1.4


1.2 1.3


3


10


5 4


3 1-8


1.8 1.6


1.5 1.4


1.3 1.2


I.I I.I


2


Noon


3-5

3-5 3


2-5 1.8


1.6 1.4


1.2 I.I


i i


I I


Noon




15-31 i -i 5
December


15-30 1-15
November


15-31 1-15
October


15-30 1-15
September


15-30 1-15
August


15-31 i-x 5
July





We see from this table that the brightest hours of the day
are between n A.M. and 2 P.M., the light during this time interval
having greater actinic power than at any other hour of the day.
Views taken between May and August at 5 A.M. or at 7 P.M.,
for instance, should be given exposures from 10 to 30 times longer
than the same subjects would require between the hours 10.30
A.M. and 1.30 P.M. for the same months.

The diagram shown on Plate XCVI represents the com-
parative lengths of exposure at different hours of the day for
the entire year. The abscissae represent the days of the year
from January ist to December 3ist, while the ordinates give
the comparative lengths of exposure. This diagram has been
constructed for so-called daylight, sunlight, and skylight com-
bined, for a station elevation of 500 feet above sea-level and at a
northern latitude of 40. The lower line, marked " noon curve,"
gives the comparative lengths of exposure (as ordinates) for all
days of the year at noon. The second curve gives similar values
for the hours n A.M. 6r i P.M., one hour from noon, etc.



346



PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.



Plate XCVII shows similar curves of comparative exposures
at a northern latitude of 50. The full-line curves correspond to
an elevation of 5000 feet above sea-level and the dotted curves
correspond for the same latitude, but at sea-level.

Experience teaches that the actinic power of light- rays ema-
nating from objects in the shade on a bright day with a deep-blue
sky is about ten times as great as at the same hour on the same
date, but in dark and threatening weather. A smoky atmosphere
reduces the actinic power of light still more; it may then be from
twenty to thirty times less than it would be on a bright clear
day at the corresponding hour and date. On a bright day with
thin fleecy white clouds in the sky it is even more intense, from
two to three times greater, than on a cloudless day at the same
hour and date.

The following table, by B.unsen and Roscoe, gives the gen-
eral change in the light intensity as it increases with the altitude
of the sun above the horizon :



Altitude of the Sun (above
the Horizon in Degrees).


Actinic Power of the
Light-rays of Sun and
Atmosphere (Combined).


Actinic Power of the
Light-rays, Diffused by
the Atmosphere (without
Sunlight),





3- 1


3-1


IO


17.1


15 *


20


5 2.6


24.7


3


Qi -9


3 1 -?


40


122.8


36-1


5


145-5


38-1


60


160. 7


39-i


70


170.8


39-6


80


176.4


39-7


90


178.1


39-7



TEST EXPOSURES AND TRIAL PLATES.

Having selected an orthochromatic plate suitable for the
work in view, the observer should make some test exposures
to ascertain the speed of the plate combined with the color-
screen under known conditions. The exposed test-plates should



COMPARATIVE 'LIGHT VALUES AND EXPOSURES. 347

be developed with the developer that is to be used for all sub-
sequent exposures. A very satisfactory way to expose these
plates is to give one plate several exposures, say four, by with-
drawing the slide from the plate-holder a quarter of its length
for each successive exposure. By allowing a quarter of a second
for each exposure, the four zones exposed on the plate will have
received exposures from one to one quarter of a second. A
second plate may be similarly exposed, only increasing the final
exposure, say to one second, when the four strips will have received
the following exposures: ij, ij, ij, and i second respectively.
All exposures should be made with the same lens stop.

After development of the two trial-plates, it may be found
that the second zone of the second test-plate, the one having
received i J seconds' exposure, may have been the correctly timed
strip. Having noted the conditions of light and atmosphere,
the hour, date, stop, subject, and whether the time of i J seconds
was given with or without the color-screen, we can, with refer-
ence to comparative exposure and comparative light- value tables
or idagrams, ascertain the time for correctly exposing a similar
plate under other conditions of illumination and atmosphere
at other hours and dates, and, if need be, using different stops.

It may have been found by experiment that a certain cor-
rectly timed plate strip required ij seconds' exposure with stop
F/II for a dark landscape at 2.30 P.M. on June 8th, and we want
to ascertain the correct exposure time for the same plate brand,
but using stop F/$2 and photographing an open landscape at
3 P.M. on August 2oth.

By inspection we find under stop F/II (page 344), in the
table of comparative exposures, for the average or dark landscape
the exposure value f , and the corresponding exposure for stop F/$2
under open landscape, i. Now, as our plate required ij seconds*
exposure for stop F/II and dark landscape it will require 2
seconds for stop F/^2 and open landscape. From the diagram,
Plate XCVTI, showing the comparative exposures at different
hours and dates, we find that if on June 8th at 1.30 P.M.



348 PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS

the illumination required i second's exposure, the same illumina-
tion on August 20th at 3 P.M. would require an exposure of ij
seconds, hence the time required for our plate and subject, on
August 2oth at 3 P.M. ? would be 2X1^ = 3 seconds.

For phototopographic purposes a rather slow, double-coated
orthochromatic plate is preferable, as it gives a wide range for
correct exposure, is less subject to halation, and the negative will
have the strength requisite for making good prints.

IV! Development of Orthochromatic Dry-plates.

All photographic plates should be carefully dusted with a
soft camel's-hair brush, both after insertion in the plate-holders
and again just before immersion in the developing-bath, to remove
all dust and foreign matter from the film surface, thus preventing
the formation of transparent irregularly shaped spots on the
negative. Immediately after immersion in the developer all
air-bells or bubbles should be removed from the film surface
by gently swabbing the submerged plate with a small tuft of
cotton.

The plate is placed, film side up, in the developing- tray and
the developer is at once poured over the plate from a gradu-
ate or pouring- vessel, with a sweeping motion to cover the entire
plate surface simultaneously, and the tray is kept gently rock-
ing to flow the liquid back and forth in a wave-like motion.

When orthochromatic, double-coated non-halation plates are
used, development should be prolonged to allow the developer
to penetrate the several layers of the film. The high lights in
negatives of the ordinary single- coated orthochromatic plates
should appear after an immersion of about 30 seconds; for double-
and triple-coated plates the developing process should be con-
tinued up to 6 and 10 minutes.

If the image flashes up too quickly, the plate no doubt has
been overexposed, and it should at once be removed from the
developer and rinsed in clear water; development may now be



DEVELOPMENT OF ORTHOCHROMATIC DRY- PLATES. 349

continued in a diluted developer to which a few drops of a 10 per
cent solution of bromide of potassium have been added as a
restraining agent. An old developer may be used to advantage
for developing overexposed plates.

When the image is rather slow in making its appearance the
exposure probably was undertimed, in which case the nega-
tive will develop up too strong with clear shadows and no details.
The latter may be brought out, in a measure, by a lengthy immer-
sion of the plate in a diluted or old developer, the bath being
kept at a temperature of not over 60. Experience and obser-
vation will best teach when the proper stage in the development
may have been reached and when the plate should be removed
from the bath. The old thumb-rule, to continue development
until the image may be dimly outlined on the back of the plate,
would lead to overdevelopment if observed for heavily coated
plates. The operator may be better guided by stopping develop-
ment as soon a? the " white portions " (the shadows of the orig-
inal) of the negative begin to change and darken. Weak nega-
tives with clear shadows are generally due to underdevelopment.
Too much density often results from development in a developing
solution too concentrated or too warm.

Since ortho and isochromatic dry-plates are extremely sen-
sitive to yellow, orange, and, to an appreciable extent, to red
light, it becomes necessary to exercise great care in using the
ordinary dark-room light in their manipulation. These plates
should be exposed only to dark (" Venetian ") " ruby light,"
covered with one or two layers of " non-actinic paper " (Deni-
son's Orange Tissue or Gold Bank Envelope Paper).
Even under the exercise of these precautions, the plate in the
developing-tray should be kept covered with a cardboard or
slab of hard rubber, except when necessary to examine the progress
of development. Only enough illumination should be permitted
to enter the dark-room or dark-tent as is absolutely necessary
to conduct the operations incident to change of plates and their
development.



PHOTOTOPOGRAPHIC METHODS AND INSTRUMENTS.

Nearly all manufacturers of photographic dry-plates and
printing-papers put up special developers, recommended for use
with their films, in the form of powders, tabloids and concentrated
stock solutions, with full directions for their application.
Amateurs preferring to make up the developing solutions directly
from the chemicals had best prepare the same in the form of
saturated stock solutions, to be mixed and diluted just before
use as needed. In this connection it is well to bear in mind
that dried granulated chemicals are far more active than equal
weights of the same in crystallized form, the weight of the latter
being partly made up of " crystallization water." For instance,
dried granular sulphite of sodium has double the strength of the
same chemical in crystals, and five parts of carbonate of sodium
in the dried state have the same chemical strength as twelve
parts carbonate of sodium crystals.

If photochemical solutions are prepared by weights and
measures, careful attention should be given to the relative strength
of their component parts. Trouble in this respect, however,



Online LibraryJohn Adolphus FlemerAn elementary treatise on phototopographic methods and instruments, including a concise review of executed phototopographic surveys and of publicatins on this subject → online text (page 27 of 33)